1. Field of the Invention
The present invention relates generally to closed slot rotor laminations or punchings for use in dynamo-electric machines, and more particularly to a lamination in which the slots are formed with generally V-shaped top ends.
2. Description of the Known Art
Rotors formed of a stack of laminations having closed slots equally spaced from one another about the periphery of each lamination are known from, for example, U.S. Pat. Nos. 2,794,138 (May 28, 1957) and 3,401,280 (Sept. 10, 1968), both having been assigned to the assignee of the present invention.
Such rotors are ordinarily used in inductive AC motors, wherein conducting members which extend axially along the rotor through aligned slots of the stacked laminations interact with a rotating magnetic field created in an air gap between the outer circumference of the rotor and the inner circumference of the machine stator. Windings embedded in slots between radially inwardly projecting teeth of the stator, adjacent the air gap, are connected to the AC mains, and the stator winding conductors pass through a certain order of the stator slots so as to cause magnetic flux in the air gap to rotate in synchronism with the frequency of the AC mains.
To obtain high efficiency during normal rotational speed conditions for the rotor, the conductive members or bars in the rotor slots should have minimal resistance. Low rotor resistance, however, results in low starting torque with a high starting current and a low starting power factor. One way to achieve an effectively high rotor resistance at motor start-up, is to employ a so-called double squirrel cage rotor bar arrangement in which two parallel conductive bars pass through each rotor slot with top bars (i.e., the bars closer to the rotor circumference) having smaller cross-sectional area and, hence, higher resistance, than bottom bars set deeper in the slots, i.e. closer to the axis of the rotor. The top bars and the bottom bars are all shorted together at the axial ends of the rotor.
By constricting the rotor slots between the top and the bottom bars in the radial direction to form a so-called neck portion of the slot, the bottom bars will have a much greater inductance in relation to the top bars and, thus, relatively little current is induced in the bottom bars at motor start-up when the air gap flux rotates at greatest speed relative to the rotor bars. The effective rotor resistance at start-up is then about equal to that of the top bars and, thus, sufficient start-up torque may be obtained. When the rotor approaches normal running speed, however, the air gap field interacts with the rotor bars at a much lower frequency so that the inductance of the bottom bars becomes less significant. The lower resistance of the bottom bars then provides greater operating frequency, the actual rotor resistance approximating that of both the bottom and top bars in parallel.
Closed slot rotors of the double cage variety conventionally have the slots in the rotor laminations formed with a generally V shaped edge at the top of the slot with the center of the V creating a narrow bridge part between the top of the slot and the outer circumference of the lamination. Further, the angle defined between each leg of the V and a line drawn perpendicular to the radial center line of the slot conventionally has been between about 40 to 45 degrees. Such angles as 40 degrees and 45 degrees for the V shaped edge and would require a narrower bridge in order to deliver the same performance, and would therefore present difficulties in machining of the closed slots at the peripheries of the rotor laminations.